I was asked about dividing perennials so found this article that will give you a time line when to divide perennials at least the summer or fall blooming plants.
How to Divide Summer or Fall Blooming Perennial Plants
By Angela England
Summer and Fall Blooming Perennials Ready to Be Divided
These lilies are ready to be divided - flush with new spring growth but not yet flowering. Dividing perennials is a great task to be done around the last frost date. It's an April gardening task in my zone. This particular clump is growing wild at the old family homestead and the plant colony is at least 75 years old. I decided to take some divisions for my own home and dug them up in April about five years ago. This slide show will show you how easy it can be!
Dig up a large clump of the plants, digging deeply as you go.
When you dig up the perennial plant you want to dig up a large clump of the plants. Dig wider around than the leaves of the plant, and dig deeper than you think you should. In this case, we dug at least twelve inches deep to make sure we preserved as many of the roots as possible. Once you shake the loose soil away from the plants you will have a clearer picture of what you're dealing with and can make sure the roots are in good condition. Never replant plants with rotting or diseased roots.
Separating the Individual Plants When Dividing Perennials
With a plant that grows by spreading like these lilies or Iris, you can separate the individual plants. Just tease apart each crown, taking care to preserve as many roots as possible. Notice these roots are very healthy looking - plump, firm, and white with no sign of disease. With a plant that grown in a large, intermingled clump like scabiosa or rudbeckia you'll want to cut the clump in half. Cut away any diseased roots, or dead and woody areas, and replant the clumps of fresh, healthy looking leaves and roots.
Dividing Lilies Blooming the Following Year
The Lilies will bloom that year with more blooms the following years.
Water your transplanted divisions more frequently than usual until the plants are better established and show signs of new growth. These lilies had several blooms the year I transplanted them, however, the following year we had several more and they were larger. Most perennials will bloom that same year, although of course, you'll have more and bigger blooms in following years. If you have perennials that benefit from being divided, you'll usually want to divide them every 2-5 years depending on the rate of growth and maximum size.
Rudbeckia and Coneflowers Benefit From Division
Both rudbeckia and Echinacea benefit from being divided periodically. These popular daisy-like perennials are hardy, long-lived perennials. They are both popular in butterfly gardens, wildlife gardens, and old-fashioned cottage gardens. A rudbeckia like the one pictured here would need to be divided once every 3 to 4 years and does best when divided in the spring when new growth is just beginning to appear.
Scabosia Needs to be Divided to Stay Healthy
Scabosia is a popular plant in butterfly gardens because of the long-season blooms and pretty color varieties available. However, the plant is a short-lived perennial that can actually die out after 3 or 4 years if it is not divided. It is one perennial that will benefit from being divided. Dig up scabosia every other year to divided it and cut the rootball in half with a large knife or spade. Trim away any dead wood and then replant each half where desired. In this way, you'll keep your scabosia plants fresh and renewed and healthy.
Salvia is an herb or garden perennial that benefits from being divided. Angela England
Salvia is a huge family of plants that includes the herb sage as well as several other varieties that are usually attractive to hummingbirds and bees. It is a popular perennial plant in a variety of garden situations but almost always benefits from being divided after a few years. Salvia can get very woody and gangly after a couple years and I've noticed that the center of the plants begins to get diseased or die out. Dividing your salvia every 3 years or so will keep it fresh and help prevent disease.
taken from https://www.thespruce.com/how-to-divide-blooming-perennial-plants-
till next time this is Becky Litterer, Becky's Greenhouse, Dougherty Iowa
Hi! My name is Becky and I am a Master Gardener. I own Becky's Greenhouse in Dougherty, Iowa.